Technical information

Using NewBIOS Serial Solutions
2) A zero value indicates that no serial chip is known to be
present for that COM port.
3) COM1 to COM8 are assumed to have the IRQ line defined
by IBM for those ports.
4) An IRQ line of -1 (or 255 since these are stored as byte
values), indicates that no IRQ line has been allocated to
the port.
IBM’s technical references for PS/2 machines includes a
list of preferred addresses and IRQ lines for COM1 to COM8.
These are:
Port Address IRQ
COM1 03F8 4
COM2 02F8 3
COM3 3220 3
COM4 3228 3
COM5 4220 3
COM6 4228 3
COM7 5220 3
COM8 5228 3
In standard PC compatibles, using ISA. AT, VESA local bus and
PCI interfaces COM3 and COM4 are usually set as follows:-
Port Address IRQ
COM3 03E8 3 ( or 4 )
COM4 02E8 3
The Power On Self Test (POST), a ROM program which is run
whenever the machine is rebooted, checks the preferred
addresses of COM1 and COM2 to see if there is a serial chip
present. If a chip is present it writes its address to the list of
ports that the ROM BIOS uses. NewBIOS reads this list, and so
in turn will recognise COM1 and COM2 at their default
addresses without any interference.
Most PC’s since 1990/92 also automatically detect the presence
of COM3 and COM4 adding them to the BIOS data table. These
too are then recognised by NewBIOS.
Chapter 4 Page 67